Pyrithione (also known as 1-Hydroxy-2-pyridinethione, 2-pyridinethiol-1-oxide, 2-mercaptopyridine-N-oxide, pyridine-2-thione-N-oxide, pyridinethione-N-oxide, 2-pyridinethione, pyridinethione, or simply “PT”) has been noted for its bactericidal and fungicidal activities. Pyrithione is a bidentate ligand that forms stable complexes with most transitional metals. Metallization of pyrithione often results in highly augmented biocidial activities. Metal salts of pyrithione, such as for example, sodium pyrithione, magnesium pyrithione, barium pyrithione, bismuth pyrithione, strontium pyrithione, copper pyrithione, zinc pyrithione, cadmium pyrithione, and zirconium pyrithione, are widely used as fungicides and bactericides in a broad spectrum of commercial products, such as metalworking fluids, lubricants, paints, cosmetics and toiletries.
Zinc pyrithione (or “ZPT”) is especially useful as a broad-spectrum anti-microbial agent and preservative. It is active against both gram-positive and gram-negative bacteria, as well as fungi and yeasts. Therefore, ZPT has been used in various personal care compositions, such as for example, anti-dandruff shampoos, hair conditioners, leave-on tonics, and anti-microbial foot powders.
Bar soap is a popular product form for cleansing. A bar soap comprising ZPT is particularly desirable for its broad-spectrum anti-microbial efficacy. Aesthetics of consumer products such as bar soaps have significant impact on the consumers' perception of the products, which will in turn determine the acceptability of the products by the consumers. However, pyrithione-containing compounds can become discolored in the presence of ferric or cupric ions, even if the ferric irons are present only in trace amounts. The metal ions can also be introduced into the soap compositions unintentionally as impurities in the raw materials used for making bar soap. Further, during manufacturing, handling or storage, various metallic parts of the manufacturing equipment, such as for example, roller mills, pipes, or nozzles, may come into contact with the soap noodles or pellets, thereby introducing metal ions into the soap composition. In some situation, such contact can be maintained for a long time (e.g. overnight to 24 hours), and at a relatively elevated temperature, thereby increasing interaction between ZPT and metal ions. The resultant discoloration may adversely affect the overall aesthetics of the bar soaps and give consumers a negative impression of the soap quality.
In the past, a number of solutions have been developed in attempt to solve the ZPT discoloration problem. For example, in U.S. Pat. No. 4,161,526, JP Patent Publication 2001-278863A, U.S. Pat. Nos. 4,482,715, 4,957,658 and 4,818,436, a number of materials including zinc-containing materials, borates, reducing agents (such as alkali metal sulfites, alkali metal bisulfites, hydrazine and the like), and HEDP have been used to address the ZPT discoloration problem. However, none of these solutions can completely eliminate or effectively reduce the undesirable discoloration in ZPT-containing bar soaps, which remains a continuing concern for manufacturers.
There is a continuing need for improved ZPT-based anti-microbial bar soaps with better color stability or enhanced resistance against development of discoloration.
Further, ZPT has been known to be unstable when solubilized. It may undergo transformation upon exposure to oxidizing species or certain transition metals, such as copper and iron. The anti-microbial effect of ZPT-based personal care compositions can therefore diminish substantially over time in environments susceptible to oxidation or metallization.
Therefore, there is also a need for ZPT-based anti-microbial personal cleansing compositions with improved and extended shelf life or enhanced anti-microbial efficacy.